1. The hemD gene, encoding uroporphyrinogen III synthase, has been located adjacent to the hemC gene at 85 min on the Escherichia coli chromosome. 2. The entire nucleotide sequence (741 base pairs) of the hemD gene is reported. 3. E. coli strains harbouring plasmics containing the hemD gene produce greatly elevated levels of uroporphyrinogen III synthase. 4. Purified uroporphyrinogen III synthase, isolated from the hemD-containing strain ST1046, has an Mr of 29,000, in close agreement with that predicted from the nucleotide sequence. 5. The existence of a hem operon is suggested.

The Escherichia coli hemD gene, encoding the enzyme uroporphyrinogen III synthase (co-synthase), was cloned into multi-copy plasmids in E. coli cells that were used to generate strains producing up to 1000 times the concentration of the synthase in the wild-type. The enzyme was purified to homogeneity from these strains in milligram amounts. The enzyme is a monomer of Mr 28,000 with an isoelectric point of 5.2 and a pH optimum of 7.8. The specific activity of the purified synthase is 1500 units/mg and the Km for the substrate, pre-uroporphyrinogen, is 5 microM. The N-terminal sequence of the enzyme is Ser-Ile-Leu-Val-Thr-Arg-Pro-Ser-Pro-Ala-Gly-, in agreement with the gene-derived protein sequence. The enzyme contains four 5,5'-dithiobis-(2-nitrobenzoic acid)-titratable groups, one reacting rapidly with the reagent and three further groups having lower reactivity. The enzyme is heat-sensitive, and during heat inactivation all four thiol groups become equally available for reaction.

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